This application is based on application No. 200-1083749 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a wiring board which is used for a frequency domain of a microwave or a millimeter wave and which can be mounted on an external circuit board, and also relates to a wiring board module formed by combining a plurality of such wiring boards with one another. The wiring board and the wiring board module of the present invention are applied, for example, to an electric wave system using a millimeter wave such as a radar for measuring the distance between two vehicles.
2. Description of Related Art
In this advanced information age, it is now under examination how to utilize as an electric wave for communication, an electric wave whose frequency is in a range from a microwave domain of 1 to 30 GHz to a millimeter wave domain of 30 to 300 GHz.
In such a high frequency system, because the frequency is high, a high frequency signal is highly attenuated. tenuated. Accordingly, a transmission line made of a low-loss material is used for every line through which a high frequency signal passes. Conventionally, there has been formed a multichip module in which high frequency elements are mounted on a wiring board made of material such as high-purity alumina which is small in dielectric loss tangent tan xcex4, and this wiring board and a connection board made of a similar material are bonded and connected to a metallic chassis.
FIG. 7 is a schematic section view illustrating the structure of such a multichip module. As shown in FIG. 7, a multichip module 60 comprises a metallic chassis 61, a lid 62, a plurality of high frequency elements 63 disposed inside of the lid 62, a connection board 64 and a waveguide tube conversion microstrip line board 65. The high frequency elements 63 are connected to the connection board 64 and/or the waveguide tube conversion microstrip line board 65 with wire bondings 68. The metallic chassis 61 has a waveguide opening, to which a dielectric window 66 is brazed for a hermetical sealing. A connection waveguide tube 67 is connected to the waveguide opening.
In such a multichip module, a number of high frequency elements are mounted on a single metallic chassis, resulting in an increase in the size of the metallic chassis. Accordingly, the metallic chassis and the lid are readily deformed, thus introducing the danger of the hermetical sealing being ruined. To form one module, it is required to connect each high frequency element to each wiring with the use of a gold wire or a gold ribbon 68. Accordingly, even a single defective connection makes the whole module defective. This adversely affects module production yield.
To solve the above-mentioned problem, it is proposed to form a module by hermetically housing a plurality of high frequency elements in a plurality of packages, respectively and by mounting such a plurality of packages on the surface of a predetermined module substrate.
When forming a wiring board module by surface-mounting, on the surface of a module substrate, wiring boards such as packages having high frequency elements mounted thereon, the wiring boards are connected to one another by a wiring layer formed on the surface of the module substrate, and signal transmission is made through this wiring layer. In such a case, when the signal frequency is high, a high frequency signal reflects at the connection parts of the wiring boards with the wiring layer of the module substrate, thus disadvantageously causing the high frequency signal to be readily attenuated.
Further, when a composite material such as a glass-epoxy resin composite material is used as the material of a module substrate, the dielectric loss tangent which causes a high frequency signal to be adversely attenuated on the surface of the module substrate. However, when a material small in dielectric loss tangent is used as the module substrate material, the cost becomes extremely expensive.
It is therefore desired to realize a wiring board and a wiring board module, both of which are low in cost and loss, and which can be used for a high frequency domain.
It is an object of the present invention to provide a wiring board arranged such that when high frequency elements are mounted thereon, signal transmission among a plurality of such wiring boards can be made without a wiring layer on the surface of a module substrate serving as an intermediary. In addition, it is an object of the present invention to provide a module substrate which is made of an ordinary, economical substrate such as a glass-epoxy resin composite material. Furthermore, it is an object of the present invention to provide a wiring board module which has excellent transmission characteristics, and uses such wiring boards.
A wiring board of the invention has a laminated waveguide tube comprising: a pair of conductor layers parallel to the top surface and the underside of at least one dielectric layer of a dielectric substrate having one dielectric layer or a plurality of laminated dielectric layers; and lateral-wall via-hole conductor groups arranged in two rows in a line direction at an interval of less than xc2xd of a signal wavelength, the conductor layers being electrically connected to each other by the lateral-wall via-hole conductor groups. In the wiring board, a terminal end of the laminated waveguide tube is exposed at a lateral side of the dielectric substrate, thus forming an exposed face. The dielectric substrate is provided at the bottom thereof in the vicinity of the exposed face with connection pads to be used for mounting the wiring board on the surface of an external circuit board.
A wiring board module of the invention comprises: two wiring boards each having the arrangement above-mentioned with their exposed faces of the laminated waveguide tubes contacted with each other; and an external circuit board on which these two wiring boards are placed, the connection pads at the bottoms of the wiring boards being connected and fixed to connection pads formed on the surface of the external circuit board.
According to the present invention, terminal ends of the laminated waveguide tubes are exposed at lateral sides of the wiring boards, and with these exposed faces contacted with each other, the wiring boards are surface-mounted on the surface of a module substrate. Accordingly, a high frequency signal of a microwave or a millimeter wave can be transmitted directly from one wiring board to the other wiring board without an intermediary of connection parts of the wiring boards to the module substrate or a wiring layer on the module substrate. This achieves a low loss high frequency transmission between the wiring boards regardless of the dielectric loss tangent and dielectric constant of the module substrate.
Further, an economical module substrate can be used, and mounting the wiring boards on the surface of a module substrate improves productivity. It is therefore possible to obtain a low-cost wiring board module.
Further, according to the present invention, a notch can be formed in both or one of the exposed faces to come in contact with each other, and is filled with adhesives having predetermined dielectric properties. This enhances the bonding properties of the wiring boards, thus further improving the transmission characteristics.